(A) Congenic CD45.1 B6 mice were lethally irradiated and reconstituted with MHC class II-deficient CD45.2 BM. Seven to eight weeks later, PBL reconstitution was tested by FCM and syngeneic chimeras then received allogeneic B10.A BMT with 3 Gy TBI and anti-CD154. (B) CD45.1 expression in WBC lineages (CD4 and B cells in the lymphocyte gate, Mac1+ cells in the granulocyte gate), representing residual recipient-derived cells. Representative (B6→B6) and (I-Aβ-/-→B6) chimeras are shown (top and bottom, respectively). A radioresistant population of CD4 and CD8 T cells (10-30%) persisted, whereas B cells, granulocytes and monocytes (not shown) were >99% donor-derived. (C) Histograms of MHC class II (I-Aβ) expression on peripheral B cells for the same two animals shown in panel A.